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Particulate Matter (PM 10 ) In the period 1997-2007, 20-50 % of the urban population was potentially exposed to ambient air concentrations of particulate matter (PM 10 ) in excess of the EU limit value set for the protection of human health (50 microgram /m 3 daily mean not be exceeded more than 35 days a calendar year). There was no discernible trend over the period (Figure 1). Nitrogen dioxide (NO 2 ) In the period 1997-2007, 13-41 % of the urban population was potentially exposed to ambient air nitrogen dioxide (NO 2 ) concentrations above the EU limit value set for the protection of human health (40 microgram NO 2 /m 3 annual mean). There was a slight downwards trend over the period (Figure 1). Ozone (O 3 ) In the period 1997-2007, 14-62 % of the urban population in Europe was exposed to ambient ozone concentrations exceeding the EU target value set for the protection of human health (120 microgram O 3 /m 3 daily maximum 8-hourly average, not to be exceeded more than 25 times a calendar year by 2010). The 62 % of the urban population exposed to ambient ozone concentrations over the EU target value was recorded in 2003, which was the record year. There was no discernible trend over the period (Figure 1). Sulphur dioxide (SO 2 ) In the period 1997-2007, the fraction of the urban population in EEA-32 member countries that is potentially exposed to ambient air concentrations of sulphur dioxide in excess of the EU limit value set for the protection of human health (125 microgram SO 2 /m 3 daily mean not to be exceeded more than three days a year), decreased to less than 1%, and as such the EU limit value set is close to being met everywhere in the urban background (Figure 1).

Key messages

Particulate Matter (PM10)

In the period 1997-2007, 20-50 % of the urban population was potentially exposed to ambient air concentrations of particulate matter (PM10) in excess of the EU limit value set for the protection of human health (50 microgram/m3 daily mean not be exceeded more than 35 days a calendar year). There was no discernible trend over the period (Figure 1).

Nitrogen dioxide (NO2)

In the period 1997-2007, 13-41 % of the urban population was potentially exposed to ambient air nitrogen dioxide (NO2) concentrations above the EU limit value set for the protection of human health (40 microgram NO2/m3 annual mean). There was a slight downwards trend over the period (Figure 1).

Ozone (O3)

In the period 1997-2007, 14-62 % of the urban population in Europe was exposed to ambient ozone concentrations exceeding the EU target value set for the protection of human health (120 microgram O3/m3 daily maximum 8-hourly average, not to be exceeded more than 25 times a calendar year by 2010). The 62 % of the urban population exposed to ambient ozone concentrations over the EU target value was recorded in 2003, which was the record year. There was no discernible trend over the period (Figure 1).

Sulphur dioxide (SO2)

In the period 1997-2007, the fraction of the urban population in EEA-32 member countries that is potentially exposed to ambient air concentrations of sulphur dioxide in excess of the EU limit value set for the protection of human health (125 microgram SO2/m3 daily mean not to be exceeded more than three days a year), decreased to less than 1%, and as such the EU limit value set is close to being met everywhere in the urban background (Figure 1).

What progress is being made in reducing concentrations of air pollutants in urban areas to below the limit values (for sulphur dioxide, nitrogen dioxide and particulate matter) or the target values (for ozone) defined in air quality legislation?

Percentage of urban population resident in areas where pollutant concentrations are higher than selected limit/target values, EEA member countries, 1997-2007

Note:The rationale for selection of pollutant and corresponding limit/target values for CSI 004 is given in the justification for indicator selection. Only urban and sub-urban background monitoring stations have been included in the calculations.

Note:The limit value is 50 µg PM10/m3 (24 hour average, i.e. daily), not to be exceeded more than 35 times a calendar year. Over the years 1997-2007 the total population for which exposure estimates are made, increases from 25 to 109 million people due to an increasing number of monitoring stations reporting air quality data under the Exchange of Information Decision.

Note:The annual mean limit value is 40 µg NO2/m3. Over the years 1997-2007 the total population, for which exposure estimates are made, increased from 54 to 113 million people due to an increasing number of monitoring stations reporting air quality data under the Exchange of Information Decision.

Percentage of population resident in urban areas potentially exposed to O3 concentration levels over the long-term objective for protection of human health, EEA member countries, 1997-2007

Note:The target value is 120 µg O3/m3 as daily maximum of 8 hour mean, not to be exceeded more than 25 days per calendar year, averaged over three years. Over the years 1997-2006 the total population for which exposure estimates are made, increases from 48 to 112 million people due to an increasing number of monitoring stations reporting under the Exchange of Information Decision.

Note:The limit value is 125 µg SO2/m3 as a daily mean, not to be exceeded more than three days in a year. Over the years 1997-2007 the total population for which exposure estimates are made, increases from 56 to 101 million people due to an increasing number of monitoring stations reporting under the Exchange of Information Decision.

PM10 in the atmosphere can result from direct emissions (primary PM10) or emissions of particulate precursors (nitrogen oxides, sulphur dioxide, ammonia and organic compounds) which are partly transformed into particles by chemical reactions in the atmosphere (secondary PM10).

For the period 1997-2007, the number of monitoring stations in some areas of Europe was relatively small and results may not be representative for all parts of Europe (Buijsman et al., 2004). Notwithstanding these limitations, it is clear that a significant proportion of the urban population (20-50 %) was exposed to concentrations of particulate matter in excess of the EU limit values set for the protection of human health (Figure 2).

The observed time series is short and the natural meteorological variability is large therefore it is not possible to draw firm conclusions on a possible trend in the data. Preliminary analyses indicate a downward change in the highest daily mean PM10 values although for the majority of stations the observed change is statistically not significant. In Figure 3, the 36th highest daily average is shown; compliance with the short-term limit value is assured when this value is below 50 microgram/m3.

Despite the likely future reductions in emissions, PM10 concentrations are expected to remain well above the daily limit values in most of the urban areas in the near future.

Nitrogen dioxide (NO2)

The main source of nitrogen oxides emissions to the air is the use of fossil fuels; road transport, power plants and industrial boilers account for more than 95% of European emissions.

In the period 1997-2007, 14-41 % of the urban population lives in cities with urban background concentrations in excess of the 40 microgram NO2/m3 limit value (Figure 4). However, it is expected that also in cities where the urban background concentration is below the limit value, limit values are exceeded at hot spots, in particular in locations with high density of traffic.

Enforcement of current EU legislation (Large Combustion Plant and IPPC Directive, Auto-Oil programme, the National Emissions Ceilings Directive (NECD) and LRTAP Convention protocols) have resulted in a reduction of nitrogen oxides (NOx) emissions. Until now this reduction has not been fully reflected in the annual averages of NOx concentrations observed at the urban background stations. Figure 5 shows a prevailingly decreasing trend.

Peak nitrogen dioxide levels occur often in busy streets in cities where road traffic is the main source. Since the introduction of catalytic converters at the end of the 1980s, their growing penetration in the car fleet and other measures have contributed to reducing emissions (-25 % since 1980 in the EU-15) [1]. The result has been a downward trend in the number of exceedances of the hourly limit value [2]. Peak levels depend on meteorological conditions; year-to-year fluctuations are 10 to 20 % or more.

In total 25 of the EEA member countries which are included in the Urban Audit have submitted information on nitrogen dioxide concentrations at 'urban background' and 'sub-urban background' stations to the air quality database 'AirBase'.

Although reductions in emissions of ozone precursors appear to have led to lower peak concentrations of ozone in the troposphere, the current target level is frequently exceeded for a substantial part of the urban population of the EEA member countries. Figure 6 shows estimates for 2007, indicating that 90 % of the urban population experienced exceedance of the 120 microgram O3/m3 level during at least one day (the long-term objective for protection of human health), while about 19 % of the urban population was exposed to concentrations above the 120 microgram O3/m3 level during more than 25 days. The target level was exceeded over a wide area (and much more than just 25 days).

Several studies have shown that ozone peak values have tended to decrease during the first half of the nineties. However, the officially reported data (available in AirBase) for a consistent set of stations over the period 1996-2001 shows hardly any variation for the 26th highest daily maximum 8-hour average. Figure 7 shows this 26th highest value; if it drops below 120 microgram O3/m3, there is compliance with the target value. Furthermore, the annual mean ozone concentrations have increased, which is in agreement with several studies.

The reductions in ozone precursor emissions that should result from enforcement of the NEC Directive and the LRTAP Convention protocols are unlikely to reduce ozone concentrations to below the current target value and long-term objective over the whole of the EEA area.

Sulphur dioxide (SO2)

Sulphur in coal and oil and in mineral ores is the main source of sulphur dioxide in the atmosphere. Up to 1960s, coal and oil combustion in large and small sources was the typical situation in many European cities, resulting in very high sulphur dioxide and PM concentrations. Since then, the combustion of sulphur-containing fuels have largely been removed from urban and other populated areas, first in western Europe and now also increasingly in most central and eastern European countries. Large point sources (power plants and industries), remain the predominate source of sulphur emissions. These sources, usually with high stacks, are most often located away from population centres.

As a result of the important reductions in sulphur dioxide emissions achieved in the last decades, the fraction of the urban population exposed to concentrations above the EU limit value has been reduced to less than 1 % (Figure 8). The reduction in sulphur dioxide peak concentrations is more clearly seen in the trend of the 4th highest daily sulphur dioxide concentration on urban stations in the period 1997-2007 (Figure 9). Compliance with the limit value for the daily mean is assured when the 4th highest concentration is below 125 microgram SO2/m3. A further decline in concentrations is expected in the coming years. However, peak concentrations above EU limit values still occur, especially close to sources and in cities. Peak levels strongly depend on meteorological conditions; year-to-year fluctuations are 10-20 % or more.

Several factors have contributed to the decrease in sulphur dioxide concentrations. The first (1985) and the second (1994) sulphur protocol under the UNECE LRTAP Convention, together with EC limit values set in the previous Air Quality Directive (89/427/EEC amending 80/779/EEC) have resulted in major European emission reductions and correspondingly decreasing ambient concentrations. Political changes in the beginning of 1990s in the central and eastern European countries connected with economic restructuring, decline of heavy industry and adoption of abatement measures on large point sources has contributed to decreasing winter smog episodes in central and western European countries. Policies and measures such as the Large Combustion Plants Directive, the IPPC Directive, standards regulating emissions from transport, the National Emission Ceilings Directive, and the reductions agreed under LRTAP Convention are expected to further reduce sulphur dioxide levels. Programmes for the reduction of sulphur emission from ships are also underway.

In total 22 of the EEA-32 member countries which are included in the Urban Audit have submitted information on sulphur dioxide concentrations at 'urban background' and 'sub-urban background' stations and the data is available in the air quality database AirBase. However, the majority of the information on sulphur dioxide concentrations results from stations in EU-15 Member States. The limit values tend to be more widely exceeded in the Central and Eastern European countries.

Indicator specification and metadata

Indicator definition

The indicator shows the fraction of the urban population that is potentially exposed to ambient air [1] concentrations of pollutants [2] in excess of the EU limit value set for the protection of human health.

The urban population considered is the total number of people living in cities with at least one monitoring station at a background location. The population data applied for the indicator derives from the Urban Audit, which is conducted at the initiative of the Directorate-General for Regional Policy at the European Commission, in cooperation with Eurostat and the national statistical offices of the 27 current Member States. Currently, the Urban Audit involve more than 620 European cities in 30 EEA member countries. The Urban Audit contains data for over 250 indicators across nine domains (e.g. demography, social aspects, environment, travel and transport).

The Urban Audit aims at a balanced and representative sample of cities in Europe. To obtain such a selection, a few simple rules are applied:

1. Approximately 20% of the national population should be covered by the Urban Audit.

5. The selected cities should be geographically dispersed within each Member State.

The selection of cities was prepared in close collaboration between the Directorate-General for Regional Policy, Eurostat and the national statistical institutes. To ensure that large and medium-sized cities are equally represented in the Urban Audit, in some of the larger Member States not all large cities could be included.

The Urban Audit works with three different spatial levels: the core city, the larger urban zone (LUZ) and the sub-city district (SCD). For CSI 004 only the the core city level is considered, which is the most important level. To ensure that this level is directly relevant to policy makers and politicians, political boundaries were used to define the city level. In many countries these boundaries are clearly established and well-known. As a result, for most cities the boundary used in the Urban Audit corresponds to the general perception of that city. Due to the highly diverse nature of political boundaries in the European Union, for some cities the political boundary does not correspond to the general perception of that city. In a few cities, Dublin for example, the political boundary of the city is narrower than the general perception of that city.

Exceedance of air quality limit values occurs when the concentration of air pollutants exceeds the limit values specified in the first Daughter Directive of the Air Quality Framework Directive for SO2, PM10 [3], NO2 and the target values for O3 as specified in the third Daughter Directive. Where there are multiple limit values (see section on Policy Targets), the indicator uses the most stringent case:

Sulphur dioxide (SO2): the daily limit value;

Nitrogen dioxide (NO2): the annual limit value;

Particulate matter (PM10): the daily limit value;

Ozone (O3): the target value.

[1] 'Ambient air' shall mean outdoor air in the troposphere, excluding work places.

[2] 'pollutant' shall mean any substance introduced directly or indirectly by man into the ambient air and likely to have harmful effects on human health and/or the environment as a whole.

Units

Percentage of the urban population in Europe potentially exposed to ambient air concentrations (in microgram/m3) of sulphur dioxide (SO2), particulate matter (PM10), nitrogen dioxide (NO2) and ozone (O3) in excess of the EU limit value set for the protection of human health.

Rationale

Justification for indicator selection

This indicator of the exposure of urban populations to air pollution focuses on sulphur dioxide, particulate matter (PM), nitrogen oxides and ground-level ozone. Sulphur dioxide (SO2) is directly toxic to humans, its main action being on the respiratory functions. Indirectly, it can affect human health as it is converted to sulphuric acid and sulphate in the form of fine particulate matter.

Epidemiological studies have reported statistical significant associations between short-term, and especially long-term exposure to increased ambient PM concentrations and increased morbidity and (premature) mortality. PM levels that may be relevant to human health are commonly expressed in terms of PM10 meaning particulate matter which passes through a size-selective inlet with a 50 % efficiency cut-off at 10 microgram aerodynamic diameter. Health effect associations for the PM2.5 fraction are even more clearly evident. Although the body of evidence concerning the health effects of PM is increasing rapidly, it is not yet possible to identify a concentration threshold below which health effects are not detectable.

PM10 in the atmosphere results from direct emissions (primary PM10) and from emissions of particulate precursors (nitrogen oxides, sulphur dioxide, ammonia and organic compounds) which are partly transformed into particles by chemical reactions in the atmosphere (secondary PM10).

Short-term exposure to nitrogen dioxide may result in airway and lung damage, decline in lung function, and increased responsiveness to allergens following acute exposure. Toxicology studies show that long-term exposure to nitrogen dioxide can induce irreversible changes in lung structure and function.

Exposure to high ozone concentration for periods of a few days can have adverse health effects, in particular inflammatory responses and reduction in lung function. Exposure to moderate ozone concentrations for longer periods may lead to a reduction in lung function in young children.

Scientific references

European Communities (2009)
Eurostat Structural Indicators related to urban air quality: 1) Urban population exposure to air pollution by ozone, and 2) Urban population exposure to air pollution by particulate matter.

OECD (2004)
OECD Key environmental indicators (KEI). Available indicator: SO x and NO x emission intensities. Medium term indicator: Population exposure to air pollution.

European Communities (2001)
Towards a Local Sustainability Profile. European Common Indicators. Methodology sheet for indicator no. A.5.

WHO (2002)
Air Quality Guidelines for Europe. Second edition. WHO Regional Publications, European Series, No. 91.

Policy context and targets

Context description

This indicator is relevant information for the current European air quality legislation related to the protection of human health in the adopted Daughter Directive for sulphur dioxide, oxides of nitrogen, particulate matter and lead in ambient air (Council Directive 1999/30/EC).

A combined ozone and acidification abatement strategy has been developed by the Commission, resulting in the Ozone Daughter Directive (2002/3/EC) and the National Emission Ceiling Directive (2001/81/EC). In this legislation, target values for ozone levels and for precursor emissions have been set.

Targets

EU limit values on concentrations of sulphur dioxide in ambient air

Two limit values have been set for the protection of human health. Both limit values had to be met by 1 January 2005.

a limit value of 125 microgram SO2/m3 as an daily average, not to be exceeded more than three times a calendar year, has been set for the protection of human health in the adopted Daughter Directive for sulphur dioxide, oxides of nitrogen, particulate matter and lead in ambient air (Council Directive 1999/30/EC, Section I of Annex I).

an hourly limit value for the protection of human health has been set at 350 microgram SO2/m3; this level may not be exceeded more than 24 times a calendar year.

EU limit values on concentrations of nitrogen dioxide in ambient air

Both limit values have to be met by 1 January 2010:

In the first Daughter Directive (Council Directive 1999/30/EC, section 1 of Annex II) an annual mean limit value for nitrogen dioxide of 40 microgram NO2/m3 has been set for the protection of human health.

In addition, an hourly limit value of 200 microgram NO2/m3 not to be exceeded more than 18 times a calendar year has been set.

EU limit values on concentrations of PM10 in ambient air

Both limit values had to be met by 1 January 2005.

a limit value for PM10 of 50 microgram/m3 (24 hour average, i.e. daily), not to be exceeded more than 35 times a calendar year is set for the protection of human health by the first Daughter Directive for sulphur dioxide, oxides of nitrogen, particulate matter and lead in ambient air (Council Directive 1999/30/EC, Annex III).

an additional limit value of 40 microgram/m3 as annual average has also been set.

EU target values on concentrations of ozone in ambient air

A combined ozone and acidification abatement strategy has been developed by the European Commission, resulting in a new Ozone Daughter Directive (2002/3/EC) and a National Emission Ceiling Directive (2001/81/EC). In this legislation, target values for ozone levels and for precursor emissions have been set.

The Ozone Daughter Directive sets a target value for the protection of human health of 120 microgram O3/m3 as maximum daily 8 hour mean, not to be exceeded more than 25 days per calendar year, averaged over three years. This target should be met in 2010.

The Ozone Daughter Directive has also set a long-term objective of 120 microgram O3/m3 as a maximum daily 8 hour average within a calendar year (not to be exceeded any day).

Directive 2001/81/EC, on nation al emissions ceilings (NECD) for certain atmospheric pollutants. Emission reduction targets for the new EU10 Member States have been specified in the Treaty of Accession to the European Union 2003 [The Treaty of Accession 2003 of the Czech Republic, Estonia, Cyprus, Latvia, Lithuania, Hungary, Malta, Poland, Slovenia and Slovakia. AA2003/ACT/Annex II/en 2072] in order that they can comply with the NECD.

Directive 2008/50/EC of the European Parliament and of the Council of 21 May 2008 on ambient air quality and cleaner air for Europe.

Methodology

Methodology for indicator calculation

Initially, AirBase stations are spatially joined with Urban Audit core cities in a Geographical Information System (GIS) in order to select AirBase stations, which fall within the boundaries of the cities that take part in the Urban Audit. The selected AirBase stations include station types classified as 'urban background' and 'sub-urban background'. It is assumed that the whole city population is exposed to this type of environment. Stations classified as 'traffic' or 'industrial' are influenced either by traffic emissions or other local emissions. Such environments are generally not representative for residential areas. The traffic and industrial stations are therefore not selected for the indicator calculations.

Based on the selection of AirBase stations air quality statistics are extracted from the database via Structured Query Language (SQL) server queries in order to update the pollutant specific indicators specified below.

The geo-political domain for calculating this indicator can be that of EEA member countries, EU Member States or individual states.

Sulphur dioxide (SO2)

For each station, the number of days with a daily averaged concentration in excess of the limit value (125 microgram/m3 as a daily mean) is calculated from the hourly values (if available) or daily values. Only time series with a data coverage of at least 75 % per calendar year are used (that is, in the case of daily values, having more than 274 valid daily values per calendar year, or 275 days if leap year). The number of exceedance days per city is obtained by averaging the results obtained for each station that falls within the city boundary.

The above procedure is repeated for all the cities covered by the Urban Audit that have fulfilled the data coverage criteria.

Depending on the number of exceedances, each city (and its population) is then classified uniquely in one of the 4 classes of exceedance days (0 days, 1-3 days, 3-6 days, >6 days).The percentage of urban population allocated to different exceedance classes is calculated by using the population in those cities assigned to each individual exceedance class and the total population in all Urban Audit cities that have fulfilled the data coverage criteria.

Particulate matter (PM10)

For each station the number of days with a daily mean concentration in excess of the daily limit value of 50 microgram/m3 is calculated from the hourly values (if available) or daily values. The selected urban stations include station types 'urban background' and 'sub-urban background'. Only time series with a data capture of at least 75 % per calendar year are used (that is, in the case of daily values, having more than 274 valid daily values per calendar year, or 275 days if leap year). The number of exceedance days per city is obtained by averaging the results obtained for each station that falls within the city boundary.

The above procedure is repeated for all the cities covered by the Urban Audit that have fulfilled the data coverage criteria.

Depending on the number of exceedances, each city (and its population) is then classified uniquely in one of the 4 classes of exceedance days (0 days, 0-7 days, 7-35 days, >35 days).The percentage of urban population allocated to different exceedance classes is calculated by using the population in those cities assigned to each individual exceedance class and the total population in all Urban Audit cities that have fulfilled the data coverage criteria.

Nitrogen dioxide (NO2)

The annual mean concentration in an Urban Audit city is calculated using the values measured at the selected stations (urban and sub-urban background). Only time series with a data capture of at least 75 % per calendar year are used (that is, in the case of daily values, having more than 274 valid daily values per calendar year, or 275 days if leap year).

The above procedure is repeated for all the cities covered by the Urban Audit that have fulfilled the data coverage criteria.

Depending on the mean concentration, each city (and its population) is then classified uniquely in one of the 4 classes of concentration (0-26 microgram/m3, 26-32 microgram/m3, 32-40 microgram/m3), >40 microgram/m3).

The percentage of urban population allocated to different concentration classes is calculated by using the population in those cities assigned to each individual concentration class and the total population in all Urban Audit cities that have fulfilled the data coverage criteria.

Ozone (O3)

For each station, the number of days with a daily maximum 8-hourly mean concentration in excess of the target value (120 microgram/m3) is calculated from the hourly values. Only time series with a data coverage of at least 75 % per calendar year are used (that is, in the case of hourly values, having more than 6576 valid hourly values per calendar year, or 6594 hours if leap year). The number of exceedance days per city is obtained by averaging the results obtained for each station that falls within the city boundary.

The above procedure is repeated for all the cities covered by the Urban Audit that have fulfilled the data coverage criteria.

Depending on the number of exceedances, each city (and its population) is then classified uniquely in one of the 4 classes of exceedance days (0 days, 0-25 days, 25-50 days, >50 days).

The percentage of urban population allocated to different exceedance classes is calculated by using the population in those cities assigned to each individual exceedance class and the total population in all Urban Audit cities that have fulfilled the data coverage criteria.

Methodology references

Uncertainties

Methodology uncertainty

The air quality data is officially submitted. Following the Exchange of Information Decision it is expected that data has been validated by the national data supplier (see Mol, W.J.A. (2008). Quality checks on air quality data in AirBase and the EoI data in 2008. ETC/ACC Working Paper December 2008. Available at http://air-climate.eionet.europa.eu/databases/airbase/ETCACC_WP_2008_Quality_checks_EoI2008_Airbase.pdf). Station characteristics and representativeness is in some cases insufficiently documented. The data is thought to be representative for the urban population in Europe as a whole. Locally, the indicator is subject to year-to-year variations due to meteorological variability.

Data sets uncertainty

Sulphur dioxide (SO2)

Strength and weakness (at data level): The air quality data is officially submitted to the European Commission under the Exchange of Information Decision. It is assumed that the air quality data has been validated by the national data supplier. Station characteristics and representativeness is often insufficiently documented. Data coverage in non EEA-32 member countries needs improvement; data availability over the period 1980-1995 needs improvement.

Reliability, accuracy, robustness, uncertainty (at data level): The number of available data series varies considerably from year to year and is for the first part of the 1990s insufficient. The data is generally not representative for the total urban population in a country. Availability of data before 1990 is too low to include in the indicator; data for non EU Member States is largely missing before 1995. Locally, the indicator is subject to large year-to-year variations due to meteorological variability. When averaging over EEA member countries this meteorologically induced variation decreases in importance provided spatial data coverage is sufficient. Due to deficiencies in information on station characteristics, the selection of urban sites (i.e. of the type 'urban background' and 'sub-urban background') might not always result in a representative selection of polluted zones. As a consequence, the indicator may be biased . The representativeness of the selection is different for different cities which reduces the comparability between cities. It is not possible at this stage to select a sufficiently large set of stations covering the entire time period since the stations with available data change from year to year.

Particulate Matter (PM10)

Strength and weakness (at data level): The air quality data is officially submitted to the European Commission under the Exchange of Information Decision. It is assumed that the national data supplier has validated the air quality data. Station characteristics and representativeness is often insufficiently documented. Geographical coverage and data availability still needs improvement. Data has been considered both from monitoring with the reference method (gravimetry) and with other methods. It is not documented whether countries have applied correction factors for non-reference methods, and if so, which factors have been applied. Uncertainties associated with this lack of knowledge may be several tens of percents.

Reliability, accuracy, robustness, and uncertainty (at data level): In some Member States the density of PM10 monitoring stations are not sufficient to give a representative picture. The number of available data series varies considerably from year-to-year and is insufficient for the period before 1997. The data is generally not representative for the total urban population in a country. Locally, the indicator is subject to year-to-year variations due to meteorological variability. When averaging over EEA-32 this meteorologically induced variation decreases in importance provided spatial data coverage is sufficient. Due to deficiencies in information on station characteristics, the selection of urban (type 'urban background' and 'sub-urban background') sites might not always result in a representative selection of polluted areas. The indicator may be biased due to insufficient representative coverage of the pollution situation. The representativeness of the selection is likely to be different for different cities which reduces comparability.

Nitrogen dioxide (NO2)

Strength and weakness (at data level): The air quality data is officially submitted to the European Commission under the Exchange of Information Decision. It is assumed that the air quality data has been validated by the national data supplier. Station characteristics and representativeness is often insufficiently documented. Data availability over the period 1980-1995 needs improvement.

Reliability, accuracy, robustness, uncertainty (at data level): The number of available data series varies considerably from year-to-year and is for the first part of the 1990s insufficient. The data is generally not representative for the total urban population in a country. Availability of data before 1990 is too low to include in the indicator; data for non EU Member States is largely missing before 1995. When averaging over EEA-32 this meteorologically induced variation decreases in importance provided spatial data coverage is sufficient. Due to deficiencies in information on station characteristics, the selection of urban sites might not always result in a representative selection of polluted zones. As a consequence, the indicator may be biased. The representativeness of the selection is different for different cities which reduces the comparability between cities. It is not possible in this stage to select a sufficiently large set of stations covering the entire time period since the stations with available data change from year to year.

Ozone (O3)

Strength and weakness (at data level): The air quality data is officially submitted to the European Commission under the Exchange of Information Decision. It is assumed that the air quality data has been validated by the national data supplier. Station characteristics and representativeness is often insufficiently documented. Data coverage in non EEA-32 member countries needs improvement; data availability over the period 1980-1995 needs improvement.

Reliability, accuracy, robustness, uncertainty (at data level): The number of available data series varies considerably from year to year and is for the first part of the 1990s insufficient. Yearly changes in indicator value may result from changes in monitoring density and/or selected cities which will influence the total monitored population. The indicator is subject to year-to-year fluctuations as it represents episodic conditions, and these depend on particular meteorological situations, the occurrence of which varies from year to year.